The You Yangs

The You Yangs are a series of low granite ridges rising up to 300 m (at Flinders Peak) above the Werribee Plains, about 50 km WSW of Melbourne. The main ridge runs roughly N-S for about 9 km, with a lower extension running for about 15 km to the west.

Granite outcrops on the eastern face of the You YangsPhotographer: Bill Birch. Source: Museum Victoria

Contrary to popular belief, the You Yangs are not the remains of a volcano. The granite that forms them was originally a mass of magma that had worked its way up into the surrounding sedimentary rocks during a period of geological time known as the Devonian, when the land surface in Victoria was several kilometres higher than today. The magma crystallised before it reached the surface, so it did not produce any volcanic activity. Instead, a very slow cooling rate allowed many large white crystals of feldspar to form. These can be seen in many of the granite outcrops throughout the ranges. In places the crystals appear to be lined up, probably because the sticky magma was still moving around when they were growing. The rock enclosing the big feldspar crystals mainly contains crystals of greyish quartz and two black minerals (hornblende and a variety of mica known as biotite).

There are also some tiny crystals of two minerals, allanite and titanite, that contain radioactive elements such as uranium and thorium. Titanite crystals have been used to calculate that the You Yangs granite solidified 365 million years ago. In many places in the granite there are dark grey clots and lumps. These are called ‘xenoliths’ and are pieces of sedimentary rock that have been caught up and baked by the magma.

The land surface has been lowered by erosion over the millions of years since the granite solidified, so it is now exposed. Because it is a hard rock, it has resisted erosion better than the rocks that surrounded it. The size and shape of the rounded tors are controlled by fractures in the granite that resulted from slight shrinkage during cooling. Weathering and erosion of the granite has formed a blanket of sandy soil that covers any contacts with surrounding rocks.

The young volcanoes

The countryside surrounding the granite ridges is a lava plain. Known as the Werribee Plain, it forms part of the vast Western District Volcanic Plains that extend from Melbourne to the South Australian border. Volcanoes began erupting lava flows about 4.5 million years ago, and the youngest eruptions are only about 10 000 years old. There are over 400 mapped craters and vents on the plains. While all these individual volcanoes are extinct, the volcanic field itself is only dormant, so that a new eruption is possible at any time.

The nearest volcanoes are the Anakies, the three low hills on the western horizon. These all have summit craters and provided lava flows for the plains south of the You Yangs. There were also flows from the low volcanoes of Bald Hill and Spring Hill to the north. When these volcanoes were active, probably between 2 and 3 million years ago, the You Yangs would have been granite islands in a sea of lava.

Other geological features

The low wooded scarp along the western skyline is the Rowsley Fault, which has been active intermittently for millions of years. This fault is the western edge of a geological structure known as the Port Phillip Basin. Over millions of years, this basin has experienced periodic flooding by the sea, which is now represented by Port Phillip Bay. At a time of high sea level, perhaps a million years or so prior to the volcanic eruptions, the You Yangs would have been granite islands in a sparkling sea. Sands and gravels containing marine fossils occur along the southern edge of the granite ridges and mark the beaches and shorelines that formed at that time.

Hi Tnesha, thanks for your question. As you will read above, the You Yangs are not the remains of a volcano, but rather a mass of magma that worked its way up into the surrounding sedimentary rocks.

John Jongsma
29 April, 2013 17:49

Please read the article, the answer lies within!!

Narelle
28 May, 2010 13:42

Hi
My daughter has to do a project on creating an emergency plan for a natural disaster that we haven't had yet. She has chosen to do a volcanic eruption from a dormant volcano in Vic. Any info you could provide would be helpful.

Hi Codie, the You Yangs faces a number of threats such as increased fire risk with drought conditions. Also being a small 'island' of vegetation in a largely cleared landscape it makes the You Yangs vulnerable to pest plant and animal invasion. This isolation from other areas of native vegetation means some species may not be able to recolonise if lost due to fire or other threats. The closeness of this park to Melbourne and its popularity with the public can also place pressures on the environment. Parks Victoria has more information about this park.

Rebecca
22 August, 2010 16:00

Hi i was just wondering what are the main Fauna and Flora at the You Yangs?
Fauna: is it just mainly birds that live there, or are there others?
Thanks :)

Hi Ross, we are not aware of any caves found in the You Yangs, but you might want to contact Parks Victoria who manage this reserve and will know for sure.

Peterjohn Walter
9 May, 2011 20:44

hi, I know the you yangs are currently closed due to the massive rainfall we had last summer, inwas wondering if you know if there is a time schedule for when it will open, specifically the mountain biking sections?

Hi Jonnathon, the Mount Rothwell Biodiversity Interpretation Centre have information on their website relating to the geology of the You Yangs and also a timeline of European involvement with the area. There is a contact email on their page; they may be able to offer more assistance if their website doesn't answer your question.

3. The Maribyrnong, Werribee, and Moorabool drainage basins : a Koorie ramble. By Ian D. Clark, (1990). It is unpublished report but available to read at the Australian Institute of Aboriginal and Torres Strait Islander Studies Library. The library number is PMS 5331. You could contact the library to see if there is an electronic version available.

Hi Tony - that would probably a question for somewhere like Melbourne Water to answer - click the hyperlinked text to go to their page on the Werribee Catchment Area, you can also contact Melbourne Water directly through that page for more information.

Madison
24 August, 2013 17:45

This was really helpful, where would i be able to find out information about the hyrdology of the You Yangs for example the streams and rivers present in the area?

Hi Madison, if you have a look at the post from Discovery Centre on the 13th May 2012 you will find a link to the Parks Vic website, this has maps of the park which show creeks and surrounding water bodies. For detailed information on the hydrology of the area you might need to make contact with either Parks Victoria or Melbourne Water.

jacinta
3 October, 2013 21:16

What is avail with regards to school excursions to the you yangs? Isthere any education centre or someone to share info with students?? Thx

Hello!
Great information, however, I was hoping you had reference or citation to the geological map shown at the bottom of the article?
I have a similar one (http://er-info.dpi.vic.gov.au/publications/maps/63360/10745_melbourne_63_geol_ed1.jpg) but not quite the one you have.
Would love to get it, thank you!

Hello, on a clear day and if the sun is in the correct position I can see a land mass from Parkdale beach. I cant always see it, is it likely to be the You Yangs, or something else? penny for your thoughts...

Hi Ronan, there's so many variables that it would be difficult to be able to say with confidence, obviously it depends on the direction you were looking, and of course your own eyesight! However, it is probably unlikely that you're seeing the You Yangs from Parkdale Beach, as they would be over 40km away at an elevation of less than 400m. It's possible that you might be able to make out parts of the Bellarine Peninsula, however.

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